Turn on dump() by running
drupal site:mode dev
Or (better) edit sites/default/services.yml, and under twig.config, set debug:true, maybe also set cache:false
https://www.drupal.org/node/1903374
Debugging with XDebug
https://www.drupal.org/docs/8/theming/twig/discovering-and-inspecting-variables-in-twig-templates